This application is made for the purpose of purchasing a modern, state-of-the-art replacement for the Physiology Department's fourteen-year-old, medium-resolution transmission electron microscope. The costs of maintaining and repairing the present instrument will likely become untenable within the next several years. At this time, the personnel of the Physiology Department Electron Microscopy Laboratory are devoting a significant percentage of their average workday either to maintenance or repair of the microscope, or to compensation--primarily through substantial use of photographic supplies--for its design failings (which relate to the achievement of consistent high resolution and a range of magnifications sufficient for the needs of the microscope users). The laboratory in which the present microscope is located is in all other respects an extremely well-equipped facility for the performance of conventional transmission electron microscopy, and its personnel--which include the Principal Investigator--are extensively educated in the field of cell biology and highly skilled in the techniques of electron microscopy. Even if the age-related failures of the machine did not exist, the limitations of the present microscope would constitute a bottleneck with respect to the research and service functions of the laboratory. This laboratory serves a large number of investigators, the majority of whom are funded by the NIH; a great deal of published research has already resulted from the existence of this laboratory as an independently functioning entity. Should the present electron microscope--which after all is the basis of the laboratory--be taken out of service, these investigators will have recourse only to a central facility, which provides instrument time on a fee-for-service basis. Though the laboratory personnel would continue to be responsible for preparation of tissue samples, the efficiency level of the laboratory (a primary function of which is to act as core facility for a Program Project) would be seriously compromised. The costs thus incurred for conventional microscopy, furthermore, would preclude use of the central facility for the other services it provides, which include scanning electron microscopy, freeze-fracture replication, and X-ray analysis; in fact, most investigators in the user group described in this application would not have sufficient funds to support the substantial amount of transmission electron microscopic research already being conducted in the Physiology EM Laboratory, were they to be charged for the use of another facility's microscope.